This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A survey typically involves deploying seismic source(s) and seismic sensors at predetermined locations. The sources generate seismic waves, which propagate into the geological formations creating pressure changes and vibrations along their way. Changes in elastic properties of the geological formation scatter the seismic waves, changing their direction of propagation and other properties. Part of the energy emitted by the sources reaches the seismic sensors. Some seismic sensors are sensitive to pressure changes (hydrophones), others to particle motion (e.g., geophones), and industrial surveys may deploy only one type of sensors or both. In response to the detected seismic events, the sensors generate electrical signals to produce seismic data. Analysis of the seismic data can then indicate the presence or absence of probable locations of hydrocarbon deposits.
A marine seismic acquisition survey typically involves one or more vessels towing at least one seismic streamer through a body of water believed to overlie one or more hydrocarbon-bearing formations. As those in the art having the benefit of this disclosure will appreciate, a “marine” survey may be performed not only saltwater environments, but also in fresh and brackish waters. The term “marine” is not limited to saltwater as used in this context by those in the art.
WesternGeco L.L.C. currently conducts high-resolution Q-MARINE™ surveys, in some instances covering many square kilometers. A survey vessel known as a Q-TECHNOLOGY™ vessel may conduct seismic surveys towing multiple 1,000 m-10,000 m cables with a cross-line separation of 25 m-50 m, using the WesternGeco proprietary calibrated Q-MARINE™ source. “Q” is the WesternGeco proprietary suite of advanced seismic technologies for enhanced reservoir location, description, and management.
Wide-azimuth (“WAZ”) towed streamer survey data is typically acquired using multiple vessels, wherein there is at least a one vessel to one offset source ratio, for example: one streamer vessel and two source vessels; two streamer vessels and two source vessels; or one streamer vessel and three source vessels. Several wide- or rich-azimuth techniques are known to the art. Cole, R. A. et al., “A circular seismic acquisition technique for marine three dimensional surveys”, Offshore Technology Conference, OTC 4864, May 6-9, 1985, Houston, Tex., described a concentric circle shooting scheme for obtaining three dimensional marine survey data around a sub-sea salt dome. Another technique was described by Cole, R. A et al., in the paper presented at the SEG conference in 1988 “Three dimensional marine data acquisition using controlled streamer feathering”, where overlapping circles acquisition is proposed.
Wide azimuth seismic surveys utilize several energy sources with very large and very different offsets. Traditionally this is achieved by utilizing a dedicated source vessel for each source that is offset from the streamer array, thus requiring a one to one ratio of towing vessels with offset sources. However, the need for large offsets and multiple vessel surveys has limited the utilization of wide-azimuth surveys.